Superbus Phase I: Accessory Loads Onboard a Parallel Hybrid-Electric City Bus

This paper describes the results from the first phase of the Superbus Project, which explores the input power trends and dependencies of the major accessories on a parallel hybrid urban transit bus. More specifically, this paper examines the elimination of both ?accessory overdrive?, where more power is delivered to an accessory than is required by the function, and ?parasitic loading?, where the accessory consumes power with no useful output. The bus was equipped with an array of sensors and a programmable data acquisition system (DAQ), and was driven on routes in Minneapolis during August and September, 2008. The accessories analyzed were the hydraulic pumps, the air compressor, the alternator, and the air conditioning system. Collection and processing methods are described, and the influence of accessory overdrive and parasitic loading are demonstrated. The average input power to the accessories was 11.0 kW when the air conditioning was off and 19.3 kW when the air conditioning was on. By removing the effects of accessory overdrive and parasitic loading, it is estimated that replacing mechanically driven accessories with their electrically driven counterparts would reduce the accessory power demand by 34% (no air conditioning) and 31% (with air conditioning). Under the somewhat conservative assumption that with the air conditioning on, 50% of the bus? fuel is consumed by its accessories, it is estimated that accessory electrification would result in a 13-15% improvement in overall fuel economy.